Device and method for controlling the contrast in an image observation system provided with an MCP image intensifier

ABSTRACT

A device for determining a control signal for an image observation system including an image intensifier and an image converter wherein the control signal is representative of the contrast range of an output signal of the image converter and with control elements for adjusting the gain of the image intensifier and the gain of the image converter in a manner whereby the control signal at a value representative of a large contrast range in the output of signal of the converter results in higher adjustment gain of the image intensifier and a lower adjustment gain to the image converter and where the control signal at a value representative of a low contrast range in the output signal of the image converter results in lower adjustment gain to the image intensifier and higher adjustment gain to the image converter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device and method for controlling thecontrast in an image observation system comprising an image intensifierprovided with a microchannel plate and an image converter which convertsthe output image of the image intensifier into a video signal or avisible image.

2. Brief Description of the Prior Art

Such systems are already known and an image intensifier intended for usein such a system and having a microchannel plate, hereinafter called anMCP for brevity, is disclosed, for example, in "Microchannel Plates"Mullard Technical publication M81-0151, Mullard, Redhill, Surrey, GB andin "Image Tubes" by Illes P. Csorba, Howard W. Sams and Co. Inc. (1985),Indianapolis, U.S.A. The converter may, for example, be a CCD (chargecoupled device) with associated read-out electronics which delivers avideo signal for display on a video screen and which, for example,observes the image of the image intensifier via an optical fibre plateor relay optics. Such a system, which therefore forms a televisioncamera system having an image intensifier placed in front of the actualimaging device is in fact called a "low light level television camera"and will therefore hereinafter be called an LLLTV camera for the sake ofbrevity. The image converter may, however, also be, for example, anoptical system (for example, a magnifying glass) or an additional imageintensifier in a night viewer.

A drawback of the known systems is that the contrast control theycontain is not adequate in those cases in which a dark scene is observedin which one or more bright points, so-called bright spots, are present.Existing contrast control systems are, in fact, based on the averagebrightness and in such a case they therefore attempt to keep the averagebrightness constant, as a result of which the dark scene portions, whichoften contain the information of interest to an observer, become stilldarker, and this is precisely what is undesirable. In addition, in thecase of a video signal, the existing contrast control system is designedto adjust back the brightness in the event of bright peaks in the videosignal, regardless of the average brightness, and this only increasesthe problem outlined.

SUMMARY OF THE INVENTION

The object of the invention is to offer a solution to this problem andfor this purpose, it provides a device of the abovementioned type whichis provided with means for determining a control signal which isrepresentative of the contrast range in the output signal of theconverter and with means of controlling the gain of the imageintensifier and the gain of the image converter in a manner such that,if the control signal has a first value which is representative of alarge contrast range in the output signal of the converter, the gain ofthe image intensifier is high and that of the image converter is low,and if the control signal has a second value which is representative ofa low contrast range in the output signal of the converter, the gain ofthe image intensifier is low and that of the image converter is high.

The invention also provides a method in which the voltage applied acrossthe microchannel plate is increased and the gain of the image converteris reduced if the image formed by the image converter has a largecontrast range and in which the voltage applied across the microchannelplate is reduced and the gain of the image converter is increased if theimage formed by the image converter has a low contrast range.

The measures according to the invention make it possible, in the mannerto be explained in more detail below, to control the contrast of theimage formed by the converter on the basis of a dark portion or thedarkest portion in the observed scene, the brighter spots in the scene,which can already often be observed even without which imageintensifier, not affecting the total gain and said brighter spots notadversely affecting the image quality for the others.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained below in more detail on the basis of anexemplary embodiment, with reference to the drawing, in which:

FIGS. 1a, b, c show three diagrams for the purpose of explaining thebehaviour of a microchannel plate for various MCP voltages and MCPimpedances;

FIG. 2 shows a block diagram of a system in which the contrast controlaccording to the invention can be used;

FIG. 3 shows a block diagram of an LLLTV system in which the contrastcontrol according to the invention can be used; and

FIG. 4 shows a diagram that shows the relationship between the contrastrange observed by an observer and the actual contrast range in theobserved scene.

DETAILED DESCRIPTION OF THE INVENTION

An MCP image intensifier is a vacuum tube, of which one side, whichreceives the image to be intensified, forms a photocathode and of whichthe oppositely situated side forms the anode. The MCP placed inbetweenis composed of a large number of hollow glass fibres which are coated onthe inside with a semiconducting layer which is capable of providingelectron multiplication. All the said fibres are connected to a voltage,as a result of which there arises in the channel a longitudinal electricfield which provides the mechanism of secondary emission with energy.

The principle of an MCP is based on the multiplication of the electronsoriginating from the photocathode, which receives the image to beintensified, by means of secondary emission.

The gain of every channel is individually primarily dependent on thestrength of the longitudinal electric field in the channel and isconsequently dependent on the applied voltage.

If the number of secondary electrons generated is small compared withthe current which flows through the semiconducting layer of the channel(the so-called standing current or, alternatively, strip current), thevoltage gradient across said layer will remain unchanged. If, however ,the number of secondary electrons generated increases to such an extentthat the output current approaches the strip current, the voltagegradient, and consequently the longitudinal electric field in thechannel will decrease. This has the result that the gain, i.e. themultiplication of electrons, starts to decrease. This phenomenon iscalled channel saturation and results in a nonlinear gain of the channelin the MCP. In general, it may be stated that the gain of an MCP islinear provided that the output current at the anode side of the MCP isless than approximately 10% of the strip current. It will be clear thatthe gain may decrease not only as a result of a large input signal butalso as a result of the impedance of the conductive layer. After all,the higher this impedance, the lower the strip current and consequently,the level of the input signal at which channel saturation starts tooccur. For the relationship between gain and input current, MCPimpedance and MCP voltage, reference is made to FIG. 1. In FIGS. 1a, band c, the current density J_(k) supplied to the input/the cathode sideof an MCP is plotted along the horizontal axis, while the anode currentdensity J_(a) obtained at the anode side of the MCP is plotted along thevertical axis. FIG. 1a shows this relationship for an MCP having a lowimpedance, curve I showing the relationship for a high voltage acrossthe MCP and curve II showing it for a low voltage, a high or low voltagecorresponding to a high or low gain factor, respectively, of the MCP.FIG. 1b shows, in an identical manner, the relationship between thecathode current density supplied and the anode current density for anMCP having a high impedance. Finally, FIG. 1c shows said relationship inthe case where the MCP voltage is kept constant, curve I showing therelationship for a high MCP impedance and curve II showing therelationship for a low MCP impedance.

The invention is based on the insight that the effect described above,which is already known per se from the abovementioned literaturereferences, can advantageously be used in combination with a correctlydimensioned gain control of the image converter, such as a CCD camera, asecond image intensifier or an optical system to reduce the effect of"bright spots" on the contrast control of the LLLTV system or imageintensifier.

The image observation system shown in FIG. 2 comprises an imageintensifier 1 provided with an MCP, a circuit 2 for controlling the gainG_(b) of the MCP by controlling the high voltage to be applied acrossthe MCP, a converter 3, which may, for example, be a camera having CCDpick-up system and provided with its own external control 4 for the gainG_(c) thereof and having an output signal from which a visible image canbe formed, for example, on a cathode ray tube. A circuit 5 isfurthermore provided for measuring the signal strength of the entireelectronic or optical image delivered by the converter 3. The outputsignal of circuit 5 is compared in a differential circuit 6 with asignal delivered by a circuit 7 which represents the brightness desiredby an observer. The output signal of the differential circuit 6 is fedto the input of the control circuit 2.

The operation of the contrast control according to the invention canbest be explained on the basis of FIG. 4. In this figure, the brightnessB of the image on the anode of the MCP image intensifier 1 is plottedalong the vertical axis, while the amount of light L_(k) from the sceneto be observed received on the cathode of MCP image intensifier 1 isplotted along the horizontal axis in the left-hand quadrant, saidquadrant in fact being identical to the diagram shown in FIG. 1b, an MCPhaving a high impedance also being used in FIG. 4 and curve II showingthe relationship between the amount of light supplied and the anodebrightness for a high voltage across the MCP, i.e. for a high gainfactor G_(BII) and curve I showing it for a low voltage, i.e. for a lowgain factor G_(BI). The right-hand quadrant of FIG. 4 shows, along thehorizontal axis, the light level L_(w) observed by the observer afterthe anode image of the image intensifier 1 has been converted by theconverter 3 into a visible image. In this quadrant, curve I gives therelationship between the light level L_(w) observed in the visible imageby the observer and the brightness B of the image on the anode of theMCP image intensifier 1 in the case where the converter 3 has a highgain factor G_(CI) and curve II gives the said relationship in the casewhere the converter 3 has a low gain factor G_(CII).

If it is assumed that the converter 3 is set to the working point W_(CI)with the gain control 4, which corresponds to a high gain G_(CI), themeasuring circuit 5 will control the setting of the image intensifier tothe point W_(BI) via the control circuit 2 for the MCP, whichcorresponds to the low gain GB_(I). As soon as the control has reachedthe stable state, the total gain G_(tot) is therefore given by:

    G.sub.totI =G.sub.CI ×G.sub.BI.

If the converter 3 is set, by means of the gain control 4, to theworking point W_(CII), which corresponds to a low gain G_(CII), themeasuring circuit 5 will control, via the controlling circuit 2, thesetting of the image intensifier 1 to the point W_(BII) whichcorresponds to the high gain G_(BII). As soon as the control has reachedthe stable state, the total gain is then given by

    G.sub.totII =G.sub.CII ×G.sub.BII.

It is true that G_(totI) =G_(totII) because the control loop consistingof the circuits 5, 6 and 2 strives to keep the brightness of the imageobserved by the observer constant and equal to the desired brightnessset with the circuit 7. However, as emerges clearly from FIG. 4, acontrast range A in the image presented, which contrast range is shownin the left-hand quadrant in this figure, results in different contrastranges in the image observed by the observer as a consequence of thenonlinear curve I in the transmission of the MCP. If G_(tot) =G_(CI)×G_(BI) the observed contrast range, is equal to B, while B≈A, while ifG_(tot) =G_(CII) X G_(BII), the observed contrast range is equal to C,where C<A.

It has been found that, to obtain an optimum contrast control, the MCPimpedance has to be chosen so that the brightness B which is producedwhen the maximum value of the "strip current" occurs is approximately inthe same order of magnitude as the maximum brightness B_(max) which canbe processed by the image intensifier/converter combination. For theimage intensifier/CCD camera combination (if the CCD camera is coupledby means of fibre optics), the MCP impedance is around 2 Gigaohms.

In the case where the converter 3 is a night vision system, theprinciple underlying the invention can be applied by placing, behind theimage intensifier 1 with the MCP, a continuously variable attenuator,for example in the form of two polarising filters which can be rotatedin opposite directions of which the setting is mechanically coupled tothe standard control present for the maximum image brightness of thesystem, such as the circuit 7 in FIG. 2. By controlling back the maximumimage brightness, as in the case of "bright spots" in the observedscene, the attenuation as a consequence of the polarising filters thenincreases and, in order to keep the average brightness constant, thegain of the MCP is increased, which results in the contrast compressionexplained above without the average brightness changing substantially.

FIG. 3 shows a variant of the block diagram of FIG. 2 for the case wherethe converter 3 is a television camera having, for example, a CCD imagesensor, the system shown forming an LLLTV camera. In FIG. 3, identicalcomponents are indicated by the same reference numerals as in FIG. 2. Inthis exemplary embodiment, the video signal which is present at theoutput of the camera 3 is fed to a circuit 8 which determines the peakvalue of the video signal and to a circuit 9 which determines theaverage value of the video signal. The output signal of circuit 9 is feddirectly to the voltage control circuit 2 for the MCP, while thedifference between the output signals of the circuits 8 and 9 isdetermined in a differential circuit 10 and fed to the input of thecircuit 4 to control the gain of the converter 3, i.e. the camera. Inthis way, the contrast range of the image presented, which isrepresented by the output signal of circuit 10, is used to adjust thegain of the camera, as a result of which, in the event of a largecontrast range such as that due to the presence of "bright spots", thecamera is, for example, set to the point W_(CII) in FIG. 4 with a lowgain, so that the MCP is set to a high gain, for example at the pointW_(BII) in FIG. 4, so that the total gain of the system remains constantbut the contrast range in the image observed decreases with respect tothe situation in which the measured contrast range is low and the camerais set to a high gain. In this way, an automatic contrast control isobtained which provides an optimum matching to the differences in thelight levels in the scene and to any bright spots present.

It will be clear that, within the scope of the invention, a large numberof variations is possible, in particular as regards the design of themeasuring circuits 5, 8 and 9. Thus, more advanced measuring circuitsthan the average value measuring circuits 5 and 9 can be used. Apossibility is to use, instead of the circuits 5 and 9, a circuit inorder to assign every image point in the image of the converter 3 aweighting factor depending on its brightness and to use the sum of saidweighting factors as an indication of the image brightness. Thus, afixed brightness level can be chosen and then a weighting factor of +1,for example, is assigned to every image point having a brightness abovesaid level and a weighting factor of -1, for example, is assigned toevery image point having a brightness below said level. By now designingthe control circuit 5 or 9 to strive for a number of image points havinga weighting factor of +1 and that having a weighting factor of -1 toremain equal, it is already possible, in this way, to take account ofthe effect of "bright spots" better than with an average value control.A further improvement of the bright spot suppression can be obtained byassigning a weighting factor of less than +1 to image points having abrightness far above the fixed brightness level.

We claim:
 1. A device for controlling contrast in an image observationsystem comprising an image intensifier provided with a microchannelplate and an image converter for converting an output image of the imageintensifier into a video signal or a visible image, which comprisesmeans for determining a control signal representative of a contrastrange in an output signal of said image converter, means for adjustinggain of said image intensifier and means for adjusting gain of saidimage converter whereby if the control signal has a value representativeof a larger contrast range in said output signal of said imageconverter, the gain of the image intensifier is adjusted higher and thegain of said image converter is adjusted lower, and whereby if thecontrol signal has a value representative of a low contrast range insaid output signal of said image converter, the gain of said imageintensifier is adjusted lower and the gain of said image converter isadjusted higher.
 2. The device to claim 1 wherein said microchannelplate is of high channel impedance.
 3. The device according to claim 1wherein said means for determining said control signal representative ofthe contrast range in the output signal of the converter determines abrightness distribution of said output signal.
 4. The device accordingto claim 3 wherein said means for determining said control signalassigns a weighting factor to every image point in said output signaldepending on a brightness level in said output signal and derives acontrol signal from a sum of said weighting factors.
 5. The deviceaccording to claim 1 characterized in that said image converter iscomprised of an imaging device converting an optical image into a videosignal.
 6. The device according to claim 5 wherein said imaging devicecomprises a CCD imaging device.
 7. The device according to claim 1wherein said image converter is an image intensifier.
 8. The deviceaccording to claim 1 and further including a controllable opticalattenuator positioned between said image intensifier and said imageconverter.
 9. Method for controlling contrast in an image observationsystem comprising an image intensifier having a microchannel plate andan image converter converting an output image of said image intensifierinto a video signal or a visible image, characterized in that voltageapplied across said microchannel plate is increased and gain of saidimage converter is reduced if the image formed by said image converteris of a large contrast range, and wherein voltage applied across themicrochannel plate is reduced and gain of said image converter isincreased if the image formed by said image converter is of a lowcontrast range.
 10. The method according to claim 9 wherein voltageapplied across said microchannel plate in the case of a large contrastrange is increased to an extent that a transmission of said microchannelplate is no longer linear.